1. Field of the Invention
The present invention relates to a photodetector and an opto-electronic integrated circuit used in optical communications systems. More particularly, the invention relates to a mesa type photodiode, and an opto-electronic integrated circuit in which a mesa type photodiode and a transistor are integrated on a common substrate.
2. Related Background Art
Photodiodes (as will be referred to simply as PD) take an important position as photodetector for optical fibers in optical communication systems. Specifically, PIN photodiodes (as will be referred to as PIN-PD) are frequently used for middle distance transmission or for small-to-middle capacity transmission, because their bias voltage is low and because high-speed PIN-PDs may be produced at a low cost. Further, avalanche photodiodes (as will be referred to as APD) having the basic structure of PIN-PD are suitable for high-speed and large-capacity transmission or for long-distance transmission and are widely used in such applications, because they have high sensitivity and high speed response characteristics. Ordinary APDs normally have the mesa type structure or the guard ring structure, because a high voltage is applied thereto. Also, there are recently proposed various opto-electronic integrated circuits in which a PD and an amplifying transistor are formed on a common semiconductor substrate.
For example, the following trial models were recently reported. An opto-electronic integrated circuit is formed by integrating a PIN-PD and three HEMTs (high electron mobility transistors) on a common semiconductor substrate (Goro Sasaki et al., IEEE Journal of Lightwave Technology, Vol. 7, No. 10, Oct. 1989 , p1510-p1514). Another opto-electronic integrated circuit is formed by integrating a PIN-PD and a plurality of HBTs (heterojunction bipolar transistors) on a common semiconductor substrate (S. Chandrasekhar et al., IEEE Photonics Technology Letters. Vol. 2, No. 7, Jul. 1990, p505-p506).
The PD was formed as follows in such opto-electronic integrated circuits. Deposited on a semiconductor substrate in the named order are an n (p) semiconductor layer in which an n-type (or p-type) impurity is added, a high-resistive semiconductor layer with no impurity added, and a p (n) semiconductor layer in which a p-type (or n-type) impurity is added. An n-type ohmic electrode is formed on the n-type semiconductor layer, and a p-type ohmic electrode on the p-type semiconductor layer.
Further, the uppermost semiconductor layer, that is, the p (or n) semiconductor layer is removed except for the portion which is to become a light receiving area.
The PD with the above structure is easy to produce because of availability of general use techniques such as the diffusion method and the etching, but disadvantageous because of a large dark current flowing between the p-type semiconductor layer and the n-type semiconductor layer. If an opto-electronic integrated circuit is constructed by integrating the PD and various transistors on a common substrate, the dark current flows into gate electrodes of a transistor to cause the shot noise, which in turn raises a problem of lowering the receiving sensitivity. In addition, a leak current might flow between elements, which increases the dark current in the PD to cause the same problem.
The opto-electronic integrated circuits of this type have a light signal detecting function. This function is achieved by detecting an increase in electric current upon incidence of a light signal, which is flowing through either one of the anode electrode and the cathode electrode of the PD (as will be referred to as a light signal detecting electrode) which is one not conductively connected to a gate electrode of a transistor. If the current flowing through the light signal detecting electrode in the state of no incident light signal, that is, the dark current, is large, a current change upon incidence of light signal becomes relatively small enough to cause an error in light signal detecting function. In the arrangements using the conventional PDs, the dark current negatively affects the light signal detecting function, causing a problem of inferiority thereof.